Embodiments of the present invention relate to control of a managed pressure drilling system.
In the oilfield/hydrocarbon industry, boreholes/wellbores are drilled into subterranean hydrocarbon reservoirs so that the hydrocarbons can be recovered. In general, a borehole is drilled through an earth formation into a hydrocarbon reservoir and the hydrocarbons are produced through the wellbore. Typically, earth formations are explored for hydrocarbons, the borehole is drilled and then completed—which may comprise lining the borehole with cement and/or casing—and then the hydrocarbons are produced from the borehole, which may require pumps to pump the hydrocarbons up the borehole. Wellbore tools may be used in the borehole, normally suspended on a wireline, to carry out operations in the borehole to provide for the construction and completion of the wellbore and/or the production of the hydrocarbons.
The drilling of a borehole is typically carried out using a steel pipe known as a drillstring with a drill bit on the lowermost end; the drill bit is normally attached to or is a part of a bottomhole assembly attached to the lower end of the drillstring. In a drilling procedure, the entire drillstring may be rotated using an over-ground drilling motor, or the drill bit may be rotated independently of the drillstring using a fluid powered motor or motors mounted in the drillstring just above the drill bit. As drilling progresses, a flow of drilling fluid is used to carry the debris created by the drilling process out of the wellbore. During the drilling procedure, the drilling fluid is pumped through an inlet line down the drillstring, passes through holes of the drill bit, and returns to the surface via an annular space between the outer diameter of the drillstring and the borehole (the annular space is generally referred to as the annulus).
In some drilling systems, as discussed in more detail below, the pressure in the borehole being drilled is controlled in order to optimize the drilling procedure and/or minimize adverse effects affecting the drilling procedure. The drilling system comprises a large dynamic system, a long tube of drill pipe or coiled tubing that is suspended and/or moved within a borehole and a borehole that is full of a fluid that may be flowing through the wellbore at the same time the drill pipe or coiled tubing is in motion. As would be expected, the drilling systems, being large dynamic systems, have resonant properties associated with them.
Drilling fluid is a broad drilling term that may cover various different types of drilling fluids. The term “drilling fluid” may be used to describe any fluid or fluid mixture used during drilling and may cover such things as drilling mud, heavily weighted mixtures of oil or water with solid particles, air, nitrogen, misted fluids in air or nitrogen, foamed fluids with air or nitrogen, aerated or nitrified fluids and. In practice, the flow of drilling fluid through the drillstring may be used to cool the drill bit as well as to remove the cuttings from the bottom of the borehole.
In conventional overbalanced drilling, the density of the drilling fluid is selected so that it produces a pressure at the bottom of the borehole (the “bottom hole pressure” or “BHP”), which is high enough to counter-balance the pressure of fluids in the formation (“the formation pore pressure”). By counter-balancing the pore pressure, the BHP acts to prevent the inflow of fluids from the formations surrounding the borehole into the borehole. However, if the BHP falls below the formation pore pressure, formation fluids, such as gas, oil and/or water may enter the borehole and produce what is known in drilling as a kick. By contrast, if the BHP is high, the BHP may be higher than the fracture strength of the formation surrounding the borehole resulting in fracturing of the formation. When the formation is fractured, the drilling fluid may enter the formation and be lost from the drilling process. This loss of drilling fluid from the drilling process may cause a reduction in BHP and as a consequence cause a kick as the BHP falls below the formation pore pressure. Loss of fluid to the formations as a result of fracturing is known as fluid loss or lost circulation and may be expensive, as a result of lost drilling fluid, and increase the time to drill the borehole. Kicks are also dangerous and the liquid and/or gas surge associated with the influx into the borehole requires handling at surface.
In order to overcome the problems of kicks and/or fracturing of the formation during drilling, a process known as managed pressure drilling (“MPD”) has been developed. In managed pressure drilling various techniques are used to control/manage the BHP during the drilling process. In MPD, the flow of drilling fluid into and out of the borehole is controlled. This means that pumps that pump the fluid into the borehole and chokes that control the flow of fluid out of the borehole are controlled to control the BHP. Additionally, gas may be injected into the drilling fluid to reduce the drilling fluid density and thus reduce the BHP produced by the column of the drilling fluid in the drilling annulus. In general, until recently, MPD techniques have been fairly crude relying on manual control of the pumps and choke.
As described above, there are many parameters that may be controlled to control the behaviour of the drilling system. For example, these parameters include the speed of rotation of the drill bit, the weight applied to the drill bit, the orientation of the drill bit and/or the like. At the same time there are also many parameters associated with the drilling fluid that may be varied, such as the pump rate of the drilling fluid, an amount of choke applied to the drilling fluid, a density to the drilling fluid and/or the like. Additionally, wireline systems may extend into a borehole and tools, sensors and/or the like may be operated in the borehole while suspended on the wireline. Operational parameters can then be associated with the operation of such wireline tools and systems. Furthermore, pumps, such as electric submersible pumps (“ESPs”) may be used in the borehole to pump drilling fluid into the borehole, to pump production fluids out of the borehole and/or the like. Changes to any of these parameters in a wellbore system may be made singly or in combination to control the drilling/wireline/wellbore/pumping process(es). Control of the parameters may be performed by a person, such as the driller, a processor and/or a person in combination with a processor.
The systems described above comprise dynamic systems in which long lengths of pipe/wireline/tubing are extended from a surface location down a borehole and the pipe/wireline/tubing may be moved therein and/or fluids may be moved through the pipe/tubing and/or the borehole. Increasingly, parameters associated with a wellbore procedure, such as drilling/wireline procedures, pumping procedures and/or the like are sensed and used to provide feedback/input into the ongoing drilling/wireline/pumping procedure. In some procedures, closed loop automation provides for automatically carrying out a wellbore procedure using measurements of the state of the system being used and/or measurements of the effect being produced by operation of the system. Moreover, in the dynamic wellbore systems described above, any motors, pumps and/or other types of machinery that are activated/operated in the borehole, such as mud motors, electric submersible pumps and/or the like will undergo a change in state during their operation and will interact with the drillstring, tubing, wireline, borehole, the column of fluid in the borehole and/or the like, when a change of state occurs.
In systems in which a state of the wellbore system and or the effect of the wellbore system are sensed during a procedure and/or in autonomous and/or semi-autonomous wellbore systems, a state of the wellbore system can be changed based upon sensed/measured properties of the wellbore, the wellbore system, an effect/output of the wellbore system and/or the like. A change of state of the wellbore system may comprise a change in pumping rate of drilling fluid, an increase in rotation speed of the drillstring/coiled tubing in the borehole, an increase in motor speed of a downhole motor, a change in operation parameters of a wellbore tool, raising a drill bit/wellbore tool from a bottom of the borehole or from contacting an earth formation, increasing weight-on-bit and/or the like. Conventionally, a change in state of the wellbore system is directed to be made as-soon-as-possible when needed based upon measured/sensed data in order to adapt the wellbore system to changes as the wellbore procedure progresses. Previously, it has been recognized that wellbore systems, because of their configuration, may be associated with resonant frequencies and, in changing a state of the wellbore system, these frequencies may be avoided or filtered from the change of state process.
As discussed above, wellbore systems, like many large dynamic systems, exhibit resonant behaviour. A drillstring in rotation, changes of pressure and flow fluctuations in the borehole annulus are examples of large dynamic systems. Often the large dynamic systems are required to move between controlled set-points, for instance changing the rotation speed of the drillstring, changing the pressure-drop across a choke on the annulus and/or the like. Making changes in these resonant systems will often result in the development of large amplitude oscillations in the system, which may take a long time to die down, thereby interfering with the wellbore procedure and/or causing damage to equipment in the borehole.
For MPD systems it is often needed to control the wellbore pressure (at least over the open-hole section of the well) within a tight window. Thus, when the pump rate is changed the choke pressure is generally also changed to counter effects of changes in the hydraulic friction in the annulus. Typically pump rates change in less than 10 seconds, and the MPD system controller operates at even shorter timescales.